Field of the Invention
The invention relates to the technical field of methods for regulating the magnetic signature of a ship.
Description of the Related Art
The signature of a ship is the set of values of the magnetic field created by the ship, evaluated in a reference plane. When it is a surface ship, this reference plane is generally horizontal, situated under the hull of the ship approximately at one times the width of the ship, or typically 10 to 30 m.
FR 2 825 803 discloses a method for regulating the magnetic signature of a ship.
In its portion for determining the signature of the ship, this method provides for acquiring a plurality of measurements of the magnetic field, using a plurality of sensors located inside the hull of the ship.
Then, from this plurality of measurements and the value of the local terrestrial magnetic field, it provides for calculating the most likely magnetization distribution on the surface of the hull using a first model that associates a magnetization distribution with a plurality of measurements of the magnetic field, and a second model that couples the magnetization at one point of the hull with the magnetization at another point of the hull.
The magnetization in question is then the total magnetization at one point of the hull, corresponding to the addition of the permanent magnetization and the magnetization caused at that point.
Lastly, the magnetization distribution thus calculated is used to estimate the value of the magnetic field created by the whole of the reference plane, i.e. the signature of the ship.
In its regulation portion, this method provides for minimizing the signature of the ship thus determined. To that end, the hull of the ship is equipped with several so-called “magnetic immunization” loops, made from a material conducting the electrical current, and the ship stores electrical power means making it possible to circulate suitable electrical currents in each of the loops independently. The circulation of a current in a loop generates a magnetic field that is superimposed on the local terrestrial magnetic field and on the field created by the magnetization distribution on the hull, knowing that the latter is modified by the magnetic field generated by the loops. Thus, by regulating the electrical currents circulating in the different loops, the signature of the ship is minimized.
The precision of this method rests on the representativeness of the plurality of measurements of the magnetic field using the plurality of sensors situated inside the hull and on the aptitude of the first and second models to precisely determine the magnetization of the hull from the measurements resulting from the sensors.
When the sensors are located inside the hull, far from it, the method allows a satisfactory determination of the magnetization of the hull corresponding to the measurements done.
However, the measurement in the air of the magnetic field created by the hull is disrupted. In fact, a ship contains secondary magnetic sources, such as the ferromagnetic masses that make up its engines. These secondary magnetic sources produce secondary magnetic fields that are superimposed on the magnetic field created by the hull. The secondary magnetic fields represent a strong disruption for the information one seeks to obtain by using the sensors. Also, the measurements are not very representative of the magnetic field resulting from the magnetization of the hull. As a result, the magnetization of the hull calculated from the measurements done inside the ship is not precisely representative of the actual magnetization of the hull. As a result, the signature of the ship thus calculated may be very different from the actual signature of the ship.
To improve the ratio of the relevant signal to the disruption, it has been considered to bring the sensors closer to the hull, to position them in the immediate vicinity thereof, and to move them away from the secondary magnetic sources as much as possible.
However, when the sensors are brought closer to the hull, the signature determined by the implementation of the method described in document FR 2 825 803 no longer corresponds to the actual signature of the ship.